The Straight Dope

Cecil, are there such things as "baa-baa-babies"? In other words, if a human male has sexual relations with a sheep, can the sheep become fertilized and deliver a quasi-human-sheepish blob, otherwise known as a "baa-baa-baby"? Several associates claim there are bottled baa-baa-babies in the labs at UTA, and that there are laws prohibiting sexual union between men and sheep. I can't say if these people are speaking from real-life experience or not, but I say it's absurd. Granted, like creatures can produce blobs among their own kind--we've all met a few--but baa-baa-babies? There are several ways of finding out the truth about this, but it's cold in Dallas this time of year [Letter arrived in December--C.A.], and besides, if I wore a wool scarf to the rendezvous, I'd just make her nervous. So what's the straight dope? --Drew Hunter, Dallas

I know, I know, I shouldn't encourage these people, but it's been a quiet summer. Besides, you ought to see how much mail I've gotten on this subject. The partner usually proposed for these unholy couplings is a chimp or gorilla, whose chromosomes are alleged to be 99 percent identical with ours. On a related matter, some wonder why in horse-donkey crossings the union of a jackass and a mare produces a mule, but a stallion and a jenny (female donkey) produce a hinny, a horse (or quadruped, at least) of a different color, so to speak.

First things first: alarming as it may seem, it's not known with certainty that humans can't interbreed with other species. Ethical considerations prevent scientists from experimenting, except for the occasional spate of Nazi and/or rural weirdness. But given the amazing variety of things humans have been known to copulate with, surely we'd have heard something, other than in the pages of the National Enquirer, if any chimp children, baa-baa babies, etc, had been born.

Scientists also aren't quite sure what makes two species incapable of interbreeding, apart from obvious geographical, behavioral, or mechanical incompatibilities, e.g., Tab A won't fit in Slot B. Merely having different numbers of chromosomes isn't enough--donkeys have 31 chromosome pairs and horses have 32, yet they can produce viable offspring.

The numbers that people throw around in this regard are deceiving. A much-publicized paper by Mary-Claire King and Allan Wilson in 1975 declared that the DNA of humans and chimps was 99 percent identical. But so what? Each human gene is made up of roughly 100 to 1,000 nucleotides, which are like the coding in a computer program. Seemingly trivial changes in this code can cause big problems. An 0.3 percent difference in a single gene, for instance, can cause sickle-cell anemia, and it's possible an equally small difference could prevent interbreeding.

Unfortunately, DNA mapping hasn't progressed to the point that scientists can say definitely which differences are important and which aren't when it comes to crossbreeding. So we don't really know why most species can't interbreed, chromosomally speaking; we just know (or think we know) they can't. Normally in such situations I urge the Teeming Millions to undertake their own research, but in this case I'd just as soon we left well enough alone.

On to the matter of hinnies versus mules, a topic on which there is much confusion in our society. Most people who have heard of hinnies seem to think they're vastly different critters from mules, but not so: they're virtually identical. The few minor anatomical differences presumably are linked to the sex chromosomes of the parents.

The only important difference (and no doubt the reason for the different names) is that hinnies are rare compared to mules. Nobody is totally sure why this is so, but a likely guess is that it's merely a question of size--it's a lot tougher for a mama donkey to keep a semi-equine bun in the oven than it is for a mama horse.

By the way, despite what you may have heard, mules (and hinnies) aren't always sterile. However, it's only the female that's occasionally fertile, never the male. I don't know that this is necessarily going to make anybody want to change their party plans, but I just thought I'd pass it along.

Art accompanying story in printed newspaper (not available in this archive): illustration/ Slug Signroino.